The present invention relates to temperature control devices and more specifically to thermostats using Proportional-Integral-Derivative (PID) controllers.
The heating and cooling of an enclosed space has become a highly developed art. However, the present high cost of energy required to run heating and cooling systems dictates that these systems operate no more than is necessary to keep the enclosed space at a desired temperature. In addition, many enclosed spaces such as residences and commercial buildings do not need to be kept at a constant temperature throughout the day and night.
Many systems have been developed to minimize the operation time of heating and cooling systems to that necessary to maintain a comfortable temperature in the enclosed space. U.S. Pat. No. 4,615,380 ('380) issued to Beckey and assigned to the assignee of the present invention, is an example of such a system and is incorporated herein by reference. The Beckey ('380) patent teaches a thermostat which includes a Proportional-Integral-Derivative (PID) controller.
The PID controller is a well known device for use in control systems to reduce the error found in a control system. In a thermostat, the PID controller serves the purpose of reducing the amount of steady state error, where the steady state error is defined as the variation of room temperature from the thermostat set point temperature over time. The PID controller calculates the amount of error between the temperature of the enclosed space and the setpoint of the thermostat, the rate of change of the error which is the derivative of the amount of error, and the sum of past errors which is the integral of the amount of error. These three signals are all multiplied by preselected constants and are then combined to produce a signal which affects when the heater or cooler is turned on or off. By looking at the amount of error, the sum of past errors, and the rate of change of error, and combining these factors into a decision on when to turn on or off a heater or cooler, the peak error and the sum of past error can be reduced in the future.
In the Beckey ('380) patent, a further method for reducing error is disclosed. The amount of overshoot is used to adjust the preselected constants used to determine the proportions of error, the rate of change of the error and the total error to be used in the feedback signal.
The Beckey ('380) patent also discloses a thermostat for varying the temperature of an enclosed space over time. A clock and memory are included in the thermostat. A microprocessor checks the current time, checks what the temperature should be at that time, and determines if the enclosed area is at the proper temperature for that time. If the enclosed area is below the proper temperature for that time in a heating application, the heater will be turned on. In a cooling application if the enclosed area is above the proper temperature for that time, the cooler will be turned on.
A typical residence temperature profile is shown in FIG. 1. The profile represents the thermostat setpoint temperature versus time. Note that the nighttime temperature is kept at about 60.degree.F. Around 5:00 a.m., a gradual warm up, in the form of a temperature ramp, is desired so that at wake up time of 7:00 a.m., the house will be at a comfortable temperature. After the house has been vacated at 9:00 a.m., the temperature is allowed to drop some. At 3:00 p.m., a gradual warming again is desired so that when the house is again occupied, it will be at a comfortable temperature. At 10:00 p.m., when the occupants are going to bed, the temperature is returned to the night time level.
A problem exists during the recovery periods in that thermostats, including those employing PID controllers, may cause cycling from on to off and off to on of the heater or cooler. This adds cost to the operation of the heater and also can cause more noise.
One attempted solution to the problem of cycling can be found in U.S. Pat. No. 4,702 413 ('413) issued to Beckey et al. The Beckey et al. ('413) patent teaches the use of a variable temperature ramp as the thermostat set point during a temperature recovery period. Once the temperature recovery period began, the heater was locked on. However, once the temperature of the enclosed area rose above the thermostat setpoint, the heater was no longer locked on. The temperature of the enclosed space was able to rise above the thermostat set point either during the temperature recovery period or after a temperature recovery period. If the temperature of the enclosed space rose above the thermostat set point during the temperature recovery, undesirable cycling would once again occur. Further, if the heater was locked on until the temperature of the enclosed space rose above the thermostat setpoint after a temperature recovery period, thermal momentum could cause the temperature of the enclosed space to overshoot the thermostat setpoint. Or, if the temperature recovery period began too late, the temperature of the enclosed space may be too low or undershoot the desired temperature at the time to end the temperature recovery period. To correct for overshoot or undershoot in either situation, the amount of overshoot or undershoot was used to adjust the slope of the ramp.
It is an object of the present invention to provide a thermostat which minimizes temperature overshoot or undershoot while preventing cycling of the heater or cooler during a temperature recovery period while still achieving the desired temperature.